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fix(core): Fn.select incorrectly short-circuits complex expressions #19680

Merged
merged 2 commits into from
Apr 1, 2022
Merged

fix(core): Fn.select incorrectly short-circuits complex expressions #19680

merged 2 commits into from
Apr 1, 2022

Commits on Apr 1, 2022

  1. fix(core): Fn.select incorrectly short-circuits complex expressions 

    In raw CloudFormation, it is possible to do the following:

```
'Fn::Select':
  - 0
  - - { 'Fn::If': ['Cond1', 'Value1', { Ref: 'AWS::NoValue' } }
    - { 'Fn::If': ['Cond2', 'Value2', { Ref: 'AWS::NoValue' } }
    - { 'Fn::If': ['Cond3', 'Value3', { Ref: 'AWS::NoValue' } }
```

Because the `AWS::NoValue`s will disappear from the array, this
will evaluate to the first condition that is true.

CDK is unlikely to generate expressions like this, but people may have
written this in CloudFormation templates. The eager short-circuiting
behavior of `Fn.select` was breaking the roundtrippability of this
template's condition cascade through `cloudformation-include`, by
unconditionally picking out the first element from the array.

We can't get rid of the short-circuiting completely (as bunch of
templates and tests may already depend on it), but we can catch
this happening and guard against it, by not short-circuiting if
we can't look into all values.
    @rix0rrr
    rix0rrr committed Apr 1, 2022
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  2. Merge branch 'master' into huijbers/fn-select-too-smart

    @mergify
    mergify[bot] committed Apr 1, 2022
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